1. Field of the Invention
This invention relates to the production of high speed steel (HSS) powder reclaimed from grinding sludge, and more particularly to such a powder suited for the manufacture of solid components by powder metallurgical processes, such as sintering.
2. Description of the Prior Art
The term "grinding swarf", swarf or grinding sludge, as herein described, refers to a mixture containing typically about 50 to 60% fine metallic particles, about 30 to 35% oil, and about 5 to 15% aluminum oxide abrasive, such as alumina. Grinding swarf is a bi-product produced during the manufacture of tools, such as, high speed steel twist drills as a result of abrasive wheel grinding. Distinction is made between this product, which has an extremely fine, mud-like consistency, and oily chips, which are much coarser.
High speed steels are a family of alloys which contain, in addition to iron and carbon, substantial amounts of the critical carbide-forming components, such as, 0-20% tungsten, 0-10% molybdenum, 0-4% chromium and 0.5-10% vanadium. These steels were originally so named because they could be used as cutting tools in high speed metal machining operations in which the tip of the tool could exhibit a dull red glow from frictional heat during use. Whereas ordinary martensitic carbon steels softened drastically at temperatures in the range of 600.degree. to 1200.degree. Fahrenheit, high speed steels exhibited considerable hot hardness, and therefore the ability to retain a cutting edge in this temperature range. These highly alloyed materials are traditionally melted in electric arc furnaces and cast into ingots which are small in comparison to those used in carbon steel production. This is done to minimize gross alloy segregation, a condition which is prone to occur during freezing of these materials during solidification, which results in coarse segregates of carbides in the finished product.
The development of the powder metallurgical approach with HSS has largely eliminated the problem of coarse carbide segregates. The powder is conventionally produced from a melt by the use of either water or inert-gas atomization jets. Each powder grain becomes, in effect, a micro-ingot, one which cools sufficiently rapidly to prevent the gross alloy segregation found in conventional ingots.
Gas-atomized powder is usually of high purity, especially with regard to low oxygen. However, the particles tend toward a spherical shape and the powder therefore exhibits low green strengths when cold compacted to form shaped items which may later be sintered. Full densification of this powder usually requires that some form of hot deformation be employed, such as, hot isostatic pressing.
Water atomized powder, on the other hand, is usually irregular in shape and can be cold pressed into components with excellent green strength. However, the powder is, unfortunately, much higher in oxygen than is gas atomized powder and must therefore be vacuum annealed in order to lower the oxide content.
Water atomized HSS powder is most frequently cold compacted into a green compact, and then sintered at high temperature under vacuum conditions. When processed in this fashion, the final sintering temperature must be carefully controlled, often to within a temperature range of .+-.1.degree. Centigrade. If the sintering temperature is too low, sintering is incomplete and voids remain in the final part. Conversely, if the sintering temperature is too high, melting and formation of coarse eutectic carbides occurs.
An example of the prior art is found in U.S. Pat. No. 3,746,518.
An example of swarf treatment is found in U.S. Pat. No. 2,394,578 to Wulff, entitled "Reclamation of Tool Steel Scrap".